import numpy as np
import matplotlib.pyplot as plt
import Image
ax = plt.subplot(111)
t = np.arange(0.0, 5.0, 0.01)
s = np.cos(2*np.pi*t)
line, = plt.plot(t, s, lw=2)
plt.annotate('local max', xy=(2, 1), xytext=(3, 1.5),
             arrowprops=dict(facecolor='black', shrink=0.05),
             )
plt.ylim(-2,2)

plt.savefig('testplot.png')
Image.open('testplot.png').save('testplot.jpg','JPEG')
plt.show()